We have been using an anion-exchange HPLC with pulsed amperometric detection (PAD) to quantify polyribosylribitol phosphate (PRP) content in vaccines containing Hib conjugates. The method is based on specific depolymerization of PRP in sodium hydroxide (0.1 N) to its monomer (ribitol-ribose-phosphate), separation of the monomer from all other components in a vaccine by Dionex's high-pH anion-exchange HPLC column (CarboPac PA1 or PA10), and quantification of the monomer by PAD. This method was successfully used to quantify the PRP content in the final containers of all four US licensed Hib conjugate vaccines and two combination vaccines which contain Hib conjugate as one of their components. The PRP content of these six vaccines was within the specifications of manufacturers. Prior to this HPLC method, there had been no accurate chemical methods available to determine the PRP content of the two combination vaccines (COMVAX and TETRAMUNE) due to the presence of interfering substances. Our work suggests that the HPLC method can be used as a universal method to quantify the PRP content in all vaccines. In addition, the method can quantify as little as 20 ng of PRP. This level of sensitivity allows that the HPLC-PAD method to be used for stability studies of Hib conjugates in vaccines since as low as 1 % degradation of PRP from the conjugates can be measured. Currently, we are applying and exploring the HPLC method to quantify the polysaccharide content of meningococcal A and C conjugate vaccines. Preliminary results indicate that the polysaccharide content of meningococcal conjugates may also be quantified by the HPLC method after acid hydrolysis of the protein-linked PSs to their respective monosaccharide components. Haemophilus influenzae type b (Hib) was a major cause of bacterial meningitis in infants and children in the United States and in many other countries till a few years ago. Introduction Hib conjugate vaccine in infants in l990 has dramatically reduced the incidents of the disease. No direct chemical assays are available to quantitate the Hib PS in the combination vaccines due to the interference of other components in these vaccines. This HPLC method provides a sensitive and reproducible method to uantitate the Hib PS content in combination vaccines.